1. Field of the Invention
The present invention concerns gearboxes of the layshaft type, in particular for automobile vehicles, comprising for the purpose of selecting gear ratios clutches that are preferably operated while under load, such as hydraulic clutches for example, replacing wholly or in part conventional dog type systems with or without synchromesh.
2. Description of the Prior Art
It is known to implement such gearboxes with a small number of gear ratios, for example two or three forward gear ratios--one of which is generally a direct drive--and one reverse gear ratio, with a primary shaft and a secondary shaft, a gear train and a clutch for each gear ratio, and a separate special gear for the reverse gear ratio.
Such gearboxes are simple, rugged and efficient and of acceptable cost and overall size, but their performance remains somewhat limited and more often than not the reverse gear ratio is somewhat perfunctorily implemented by means of a dog type system.
Similarly, it is known to implement gearboxes with a larger number of gear ratios, for example four forward gear ratios and two or four reverse gear ratios, by combining two ranges of two gear ratios and one reversing unit, or with three forward gear ratios and three reverse gear ratios, or again with five forward gear ratios and one reverse gear ratio, etc.
Such gearboxes are still rugged and efficient but entail multiple layshafts and clutches so that their complexity, cost and overall size increase out of proportion to the number of gear ratios if an adequate level of performance is to be achieved. Also, in dual range gearboxes changing the range entails a shifting sequence that is no longer a single operation but a double operation--that is to say involving the simultaneous control of four clutches instead of two--resulting in much greater variations in inertia and much more serious problems in controlling the shifting, while the steps in the gearbox are precisely the same in both ranges, imposing gear ratio distributions that are often inappropriate.
Finally, and in a general way, these known gearboxes do not have any braked neutral setting, that is to say a combination in which the input to the gearbox is free and the output is immobilized, corresponding to the specific case where the ratio between the output speed and the input speed is equal to zero. The advantages of this type of combination, which is lacking in the types of gearbox under discussion, are specifically to permit immobilization of the vehicle with the motor running, notably during momentary stops imposed by traffic conditions, without needing to constantly apply the brakes of the vehicle, to guarantee good control over and continuity of changes between the forward gear ratios and the reverse gear ratios on reversing, and so on, all these factors tending to increase significantly driver comfort.
The object of the present invention is thus a gearbox retaining the original qualities of simplicity, ruggedness and efficiency of the known gearboxes with few gear ratios mentioned above, but also featuring, with a minimum of components and acceptable cost and overall dimensions, an increasing number of forward and reverse gear ratios that are always well distributed, a braked neutral setting in addition to the conventional unbraked neutral setting, and shifting sequences that always entail single operations only from one end to the other of the range--that is to say from the highest reverse gear ratio to the highest forward gear ratio--lending itself perfectly to any kind of control system.